Viruses, Viroids, and Prions: Structure, Classification, and Comparison with Bacteria

Chapter 13: Viruses, Viroids, and Prions

General Characteristics of Viruses

Viruses are unique infectious agents that differ significantly from cellular organisms such as bacteria. They possess distinct structural and functional properties that define their role as obligate intracellular parasites.

• Obligatory Intracellular Parasites: Viruses require living host cells to multiply. They cannot reproduce independently outside a host cell.

• Genetic Material: Viruses contain either DNA or RNA as their genetic material, but never both.

• Protein Coat: The genetic material is enclosed within a protein coat called a capsid.

• Lack of Ribosomes: Viruses do not possess ribosomes and therefore cannot synthesize proteins independently.

• No ATP-Generating Mechanism: Viruses lack metabolic machinery for energy production.

• Additional info: Some bacteria, such as Chlamydia and Rickettsia, also require living host cells to multiply, but they are cellular organisms and differ from viruses in other key aspects.

Viruses and Bacteria Compared

Viruses and bacteria differ in several fundamental ways. The following table summarizes their main differences:

Characteristic	Typical Bacteria	Rickettsias/Chlamydias	Viruses
Intracellular Parasite	No	Yes	Yes
Plasma Membrane	Yes	Yes	No
Binary Fission	Yes	Yes	No
Pass through Bacteriological Filters	No	No	Yes
Possess Both DNA and RNA	Yes	Yes	No
ATP-Generating Metabolism	Yes	Yes/No	No
Ribosomes	Yes	Yes	No
Sensitive to Antibiotics	Yes	Yes	No
Sensitive to Interferon	No	No	Yes

Key Point: Viruses are fundamentally different from bacteria in their structure, replication, and response to treatments.

Host Range of Viruses

The host range refers to the spectrum of host cells that a virus can infect. This range is determined by specific interactions between viral attachment sites and host cell receptors.

• Specificity: Most viruses infect only specific types of cells within one host species.

• Attachment Sites: Viral proteins must bind to specific receptors on the host cell surface. For example:

  • HIV infects CD4+ T cells.

  • Hepatitis C virus binds to LDL receptors on hepatocytes.

• Bacteriophages: These are viruses that infect bacteria. They are also called "phages" and range in size from 20 nm to 1000 nm.

Example: The specificity of HIV for CD4+ cells explains its role in immune system dysfunction in AIDS.

Additional info:

• Size Comparison: Viruses are much smaller than most bacteria and can pass through filters that retain bacteria.

• Applications: Understanding host range is crucial for developing antiviral therapies and predicting viral transmission.